When you enter the area where you have to fight the Compys, three boulders will fall -- one blocking the entrance, one blocking the exit and one in the middle. When all the Compys appear, lure them in front of the middle boulder. Use the Tranquilizer to paralyze them in front of the middle boulder. Then, use the Flare Gun to shoot the boulder so it runs over all the Compys in its way. The boulder will return each time. Repeat this until all the Compys have been defeated.